Salt (sodium chloride) plays an important role in seasoning and processing of foods and beverages with its activities to give a taste to foods and beverages and to improve the preservability and properties of foods and beverages. In particular, salt gives foods and beverages a taste which makes us feel delicious (a salty taste), and sodium and chlorine, which are the constituents of salt, are essential nutrients for the human body.
However, the excessive intake of sodium, which is a constituent of salt, is considered to be a risk factor which causes lots of health problems such as heart diseases (e.g., hypertension) and vascular diseases. There exists a strong demand, not only in Japan but also in other developed countries, for reduction of the intake of salt, specifically, sodium, with the increase in the number of aged people, who are prone to these diseases.
The simplest way of cutting the salt intake is to reduce the amount of salt to be used in seasoning and processing of foods and beverages. However, when the amount of salt contained in foods and beverages, whether home made ones or processed ones, is reduced by 10% or more, their taste is generally spoiled.
Known methods of reducing the intake of salt, specifically sodium, without spoiling a salty taste, which are generally called methods of salt-cut, include methods in which substances which themselves have a salty taste (hereinafter referred to as salt substitutes) are used and methods in which substances which themselves do not have a salty taste, but enhance a salty taste when used in combination with salt (hereinafter referred to as salty taste enhancing substances) are used.
Examples of the known salt substitutes include potassium salts, ammonium salts, basic amino acids, peptides comprising basic amino acids, and alkali metal salts of gluconic acid.
Potassium salts have the defect that they have not only a salty taste, but also a bitter taste, which leaves a characteristic after taste. For the purpose of solving this problem, the following have been proposed: a composition comprising dihydrochloride of basic amino acid, potassium salt or ammonium salt of glutamic acid and potassium chloride; a mixture of whey mineral and potassium chloride; a composition comprising glycine ethyl ester hydrochloride or tryptophan ethyl ester hydrochloride and potassium chloride; a composition mainly comprising potassium chloride and magnesium salt and further comprising lysine hydrochloride, etc.; and a composition comprising potassium chloride, common salt and citrate, in which the sodium/potassium ratio is 1 or lower.
Examples of the ammonium salts include a composition comprising potassium salt, ammonium salt and acidic choline salt, and a composition comprising common salt and capsuled ammonium salt.
As to the basic amino acids, the following are known: a composition comprising lysine succinate, lysine succinic acid monohydrate or ornithine adipic acid monohydrate; a composition comprising succinate of a basic amino acid; a composition comprising hydrochloride of a basic amino acid, 5′-nucleotide, a sweetening component and sodium citrate; and a composition comprising lysine succinate, etc.
Examples of the peptides comprising basic amino acids include ornithyl-β-alanine, lysyl glycine, ornithyl glycine, ornithyl taurine and lysyl taurine.
An example of the alkali metal salts of gluconic acid is potassium salt of gluconic acid.
The salty taste enhancing substances can not be substituted for salt, but enable salt-cut by enhancing the salty taste of salt to reduce the amount of salt to be used.
Examples of the known salty taste enhancing substances are as follows: peptides obtained by hydrolyzing collagen having a molecular weight of 50,000 dalton or less (Japanese Published Unexamined Patent Application No. 3766/88); thaumatin, which is a sweetening protein (Japanese Published Unexamined Patent Application No. 137658/88); a degradation solution obtained by digestion of a mixture of black koji produced with Aspergillus niger having citric acid-producing ability and yellow koji produced with Aspergillus oryzae (Japanese Published Unexamined Patent Application No. 53456/90); cetyl pyridinium salt, which is a cationic surfactant, alone or a mixture of cetyl pyridinium salt and a basic amino acid such as arginine or lysine (PCT National Publication No. 502517/91); saturated aliphatic monocarboxylic acids having 3-8 carbon atoms (Japanese Published Unexamined Patent Application No. 184326/93); an equimolar mixture of arginine, which is a basic amino acid, and aspartic acid, which is an acidic amino acid (U.S. Pat. No. 5,145,707); hydrolyzates of proteins such as egg white protein, gelatin, soybean protein, wheat protein, corn protein, fish protein, milk protein and meat protein (Japanese Published Unexamined Patent Application No. 289197/95); and trehalose (Japanese Published Unexamined Patent Application No. 66540/98).
It is described that enzymatic degradation products of proteins have salty taste enhancing activity in the above Japanese Published Unexamined Patent Application No. 3766/88 and Japanese Published Unexamined Patent Application No. 289197/95. In Japanese Published Unexamined Patent Application No. 3766/88, it is described that collagen hydrolyzate has salty taste enhancing activity, but this activity is characteristic of collagen hydrolyzate and is not observed with soybean protein hydrolyzate or milk protein hydrolyzate.
The above Japanese Published Unexamined Patent Application No. 289197/95 discloses the use of protein hydrolyzates as salty taste enhancing substances. However, it is also described that the active ingredient having the salty taste enhancing activity of the disclosed invention is free basic amino acids such as arginine and lysine formed by hydrolysis of proteins.
As described above, a number of methods using salt substitutes or salty taste enhancing substances have been proposed as salt cutting methods. However, none of the methods of salt-cut so far developed is satisfactory in respect of taste, effect, economy, safety, or the like, and low salt foods are not generally available. There exists a strong demand for a method of salt-cut overcoming the above problems.
Numbers of studies have been made on the taste of peptides. Dipeptides and tripeptides containing glutamic acid or aspartic acid have been isolated as peptides having umami from a plastein reaction product obtained by treating a pepsin degradation product of soybean protein isolate with chymotrypsin [Agr. Biol. Chem., 36, 1253 (1972)]. The result has been confirmed using synthetic peptides [Agr. Biol. Chem., 37, 151 (1973)]. Peptides having a molecular weight of 1,000 or less have been obtained by fractionation of an enzymatic degradation product of fish meat protein concentrate, and then further fractionated into acidic peptides, neutral peptides and basic peptides using an ion exchange resin, and it has been revealed that the acidic oligopeptide fraction has umami [Agr. Biol. Chem., 37, 2891 (1973)]. Further, the constituent peptides of the acidic oligopeptide fraction have been identified [J. Agric. Food. Chem., 23, 49 (1975)].
There are some reports that acidic peptides of low molecular weight have a salty taste. Delicious peptide is an umami peptide consisting of 8 amino acids which has been found in a beef soup. In the course of the research on the structure-activity relationship of the peptide, it has been found that basic dipeptides and acidic dipeptides have a salty taste [Agr. Biol. Chem., 53, 319 (1989)]. It has also been revealed, with regard to these acidic peptides and similar acidic peptides, that 4 kinds of dipeptides and 8 kinds of tripeptides consisting of aspartic acid and/or glutamic acid have a salty taste as well as umami, and further an acidic peptide consisting of 5 amino acids which is a constitutive partial peptide of the delicious peptide has been reported to have a salty taste [Biosci. Biotech. Biochem., 59, 689 (1995)].
There is also a report that peptides in an enzymatic degradation product of a protein have a salty taste. It is reported that a fraction of 500-1000 molecular weight obtained by treating gluten, which is wheat protein, with actinase, which is a proteolytic enzyme, and then deamidating the degradation product with hydrochloric acid, followed by fractionation, has a salty taste as well as a sweet taste, a sour taste, a bitter taste, an astringent taste and umami. However, it is reported that the addition of this fraction to soup stock made from boiled and dried sardines significantly strengthens umami alone, but does not bring about a significant difference with respect to the other tastes such as sweet taste and salty taste [Journal of Home Economics of Japan, 45, 615 (1994)].
Acidic peptides have been isolated from an enzymatic degradation product of soybean protein as umami peptides, and some of them have been reported to exhibit a salty taste by themselves or in the presence of inosinic acid [Biosci. Biotech. Biochem., 63, 555 (1999)].
There is also a report that acidic peptides mask the bitter taste of peptides [J. Food Sci., 40, 367 (1975)].
As stated above, lots of researches have been made on peptides, specifically acidic peptides, in respect of their own tastes and their effects on other tastes such as umami and bitter taste. They have revealed that peptides have umami, a salty taste, etc. and mask a bitter taste. However, it is not known that an enzymatic degradation product of a protein which mainly comprises peptides obtained by subjecting the protein to hydrolysis and/or deamidation enhances the salty taste of salt. In connection with the salty taste enhancing activity of a peptide having a salty taste, the strength of the salty taste of a solution of salt and ornithyl-β-alanine was evaluated, and it was confirmed that the salty taste of each solute independently appeared and the strength of the salty taste of the solution was merely additive, but not synergistic. That is, it has been revealed that ornithyl-β-alanine does not have the activity to enhance the salty taste of salt [J. Agric. Food. Chem., 38, 25 (1990)].
As described above, it has not been known yet that acidic peptides have salty taste enhancing activity. Additionally, commercially available enzymatic hydrolyzates such as seasonings and nutritious food materials obtained by enzymatic degradation of proteins do not have salty taste enhancing activity.